Metal working fluids are commonly used for purposes of cooling and/or lubricating and/or cleaning metal during metal working processes such as cutting, grinding, forming, rolling, and the like. Fluids available from commercial sources for such purposes generally include oil-in-water emulsions and non-petroleum water soluble (synthetic) compositions and synthetic emulsions or combinations thereof. Emulsified oil type metal working fluids typically comprise about 1-10% by weight of oil in combination with a small amount of an emulsifier, such as hydrogenated animal fat, sodium sulfonate complexes and the like, and about 90% or more by weight of water. Synthetic metal working fluids on the other hand typically comprise mixtures of a water miscible organic polyamine such as mono, di, and triethanolamines in combination with borates and suitable wetting agents. As used herein, unless otherwise specifically referenced, the term "metal working fluid" includes emulsified oil, synthetic composition, and synthetic emulsion type fluids and blends thereof.
Depending upon the particular metal working process, the metal working fluids characteristically become contaminated over a period of time with foreign matter such as ferrous or non-ferrous metal ions, dirt, bacteria, tramp oil, and other foreign particles. Tramp oil is a general term describing, hydraulic, lubricating and cutting oils used in various machining processes that by reason of leaks, drippings and the like enter into and contaminate the metal working fluid. The presence of tramp oil in metal working fluids has been found to be detrimental to the metal working process in that it enhances bacterial growth, smoking, and contributes to heat build up which ultimately lessens tooling life.
Although it has been common practice in the past to continuously recycle metal working fluids during the metal working process and to utilize various means to remove such contaminants in an attempt to prolong the useful life of the fluids and improve the life of tooling used in conjunction therewith, such systems have not been altogether satisfactory. Up until the time of the present invention, the principal methods of clarifying metal working fluids have been by singular or combined use of settling tanks, filters centrifuges and froth floation. Each, however, has proven to be unsatisfactory under actual manufacturing conditions which often require clarification of large volumes of contaminated fluid that are continuously recycling. Although useful for some purposes, settling tanks are generally unable to remove smaller particles that have a specific mass the same as or lighter than the fluid itself and are often too slow and/or to large to keep up with manufacturing requirements. Filters have been found to be unsatisfactory due to clogging by the contaminants in addition to expensive maintenance requirements and centrifuges have been found to be too costly due to high initial expense in addition to costly maintenance requirements.
Froth flotation has been used for many years to clarify waste water and fluid mediums other than metal working fluids. Examples of United States patents covering froth flotation of fluid mediums other than metal working fluids include: U.S. Pat. Nos. 2,153,449 (clarifying viscose rayon); 2,335,209 (removing paper pulp from white water); 2,347,147 (treatment of paper mill waste water); 2,352,365 (removal of oils and fats from emulsions); and 2,274,658 (removing gases from rayon spinning both), the disclosures of all of which are included herein by reference. In all of the above, Fagergren type mixers of the type disclosed in U.S. Pat. Nos. 1,963,122 or 2,101,331 are recommended which, while rotating at relatively high speed, are able to induce air into the fluid to produce froth which floats to the top of the fluid. However, unlike metal working fluid, little concern is given to the amount of froth produced or the loss due to the frothing of the fluid medium itself.
Froth flotation is proported to be useful in clarifying metal working fluids in U.S. Pat. No. 2,578,040, the disclosure of which is incorporated herein by reference. Such, however, has been found not to be the case, particularly in clarifying emulsion type metal working fluids for, like the other fluid mediums described above, the vigorous mixing and induction of air into the metal working fluid has been found to result in a substantial loss of emulsion forming at least part of the froth which is subsequently removed by skimming or other suitable means. In addition to being unsuitable for clarifying metal working fluids in a swift and economical manner, Froth flotation is also extremely sensitive to the hardness and variations in the hardness of the fluid being clarified which, dependent upon the hardness, necessarily produces varying amounts of froth which in turn results in the problem of having to continuously change the penetration depth of the skimmer or condition of the other means being used to remove froth as well as resulting in substantial increases in the amount of froth for mediums having a low hardness level.
Pumping (inducing) air directly into contaminated waste water without the vigorous mixing and turbulence of froth flotation has also been found to be unsatisfactory in clarifying contaminated metal working fluids for, like froth flotation, the amount of froth produced is unsuitable due to the loss of expensive metal working fluid sustained in removing the froth.
Like waste water treatment, various chemicals may be added to contaminated metal working fluids to enhance the removal of the contaminants and/or maintain desirable properties associated with the fluid.
Examples of methods for providing metal working fluids that utilize a water soluble form of a tertiary amine in order to apply a corrosion resistant film to the metal being formed or a sequestering agent for prolonging the life of the fluid are disclosed respectively in U.S. Pat. Nos. 2,917,160 and 3,365,397, the disclosures of which are incorporated herein by reference.
Other examples of methods for inhibiting the growth of bacteria with certain chemical additions and clarifying metal working oils by use of a trimerized polycarboxylic acid are disclosed respectively in U.S. Pat. Nos. 3,240,701 and 3,450,627 the disclosures of which are incorporated herein by reference. Specific methods for separating foreign matter from metal working fluids are disclosed respectively in U.S. Pat. Nos. 3,408,843; 3,618,707; 3,750,847; and 4,033,866, the disclosures of which are incorporated herein by reference.
An example of a method utilizing cationic electrolytes for treating aqueous solutions of synthetic water soluble lubricants containing suspended particles can be found in U.S. Pat. No. 3,928,784, the disclosure of which is incorporated herein by reference. The method disclosed is essentially a batch process in that it requires that the suspension stand for at least two minutes to allow flocculation and settling of the particles prior to their removal.
Other examples of specialized chemical compositions and methods for removing suspended foreign particles from metal working fluids are disclosed in U.S. Pat. Nos. 3,563,895; 3,634,243; and 4,146,488, but, as in the case of the methods previously described, none, prior to the present invention, has ever been used in conjunction with a process for removing contaminant oil and foreign matter from metal working fluids with little or no foaming in a continuous recycling process that is able to operate over long periods of time without the need for costly shutdown and maintenance problems heretofore described.
Like induced air and froth flotation, saturated dissolved air flotation, has been used to advantage in the past in clarifying waste water prior to its discharge into a stream, pond or lake. An example of the use of saturated dissolved air in combination with a vacuum to remove fibers from a water mixture is disclosed in U.S. Pat. No. 1,717,223 and the use of saturated dissolved air in conjunction with an agitator for clarifying waste water is disclosed in U.S. Pat. No. 2,248,177, the disclosures both of which are included herein by reference. However, it was not thought prior to the present invention that saturated dissolved air flotation would be a suitable method for removing contaminants in a continuous recycling process from metal working fluids, particularly oil emulsion type metal working fluids, for fear that the saturated dissolved air would promote foaming and would remove or otherwise adversely affect the emulsion. An example showing that dissolved air will remove emulsions from waste water is disclosed on page 40 in an article entitled "Air Flotation Treatment of Refinery Waste Water" by J. L. Steiner et al published in Chemical Engineering Practice in December of 1978. An example of the fact that up until the time of the present invention those ordinarily skilled in the art of metal working fluids did not appreciate or apperceive that saturated dissolved air flotation could be effectively used to clarify metal working fluids without detriment to the fluid is evidenced by the conspicuous absence of any reference thereto in an article entitled "Lubricant Conditioning and Disposal" in Chapter 30 of the "Standard Handbook of Lubrication Engineers" published by McGraw-Hill Inc. and copyrighted in 1968.
Up until the time of the present invention, manufacturers of saturated dissolved air equipment were specifying saturated dissolved air equipment that could only be used to advantage in removing contaminants from waste water with no suggestion that it could be useful in removing foreign matter from metal working fluids in a continuous recycling process in which the fluid could be continuously re-used rather than discarding it as waste sufficiently clarified to meet environmental standards. Typical examples of such teaching is disclosed in a publication by Cincinnati Milicron Company numbered PC 658 entitled "Central Filteration Systems for Cutting and Grinding Fluids" and in a publication by Komline-Sanderson Company covering a sales meeting of 1978 entitled "Dissolved Air Flotation Program".
In contrast to the various methods described above, it has been found that the process of the present invention substantially increases the life of the tooling being utilized in the particular metal working operation as well as eliminating the need to use additional filters commonly utilized for the removal of foreign matter suspended in the metal working fluid where desired. In addition, the process of the present invention by the effective removal of tramp oil has been found to provide a marked reduction or elimination of misting of the fluids commonly associated with prior art metal working fluid recycling processes so as to substantially reduce the potential of dermatitus, the inhalation of oil mist, and fire hazards commonly associated with such processes as well as causing the effective rapid removal of foreign matter and contaminant oil from metal working fluids, particularly oil emulsion type fluids, with little or no foaming and without the need for prior chemical treatment to break up the emulsion.